/Engine/Source/Runtime/GeometryCore/Public/DynamicMeshEditor.h
Find "new" normal for input normal element underIndexmapping, or create new if missing
index of duplicate normal in given normal layer
int FindOrCreateDuplicateNormal ( int ElementID, int NormalLayerIndex, [FMeshIndexMappings](API\Runtime\GeometryCore\DynamicMesh\FMeshIndexMappings) & IndexMaps, [FDynamicMeshEditResult](API\Runtime\GeometryCore\FDynamicMeshEditResult) * ResultOut )